1. Field of the Invention
The present invention relates to the use of a focused ion beam (FIB) system to remove material or deposit material in an automated manner, particularly in modification of a semiconductor device.
2. The Prior Art
A semiconductor device consists of layers of different materials. The top layers of the device structure, up to 5 or more layers, are involved with the interconnections between cells. The interfaces at which these layers meet are referred to as material boundaries; FIG. 1 shows for example a structure 100 having an upper layer 105 and a lower layer 110 which meet at a boundary 115. When a focused-ion beam is used to mill away material from a semiconductor device, it is necessary to determine that the desired material boundary has been reached so that the milling process may be terminated at the desired milling depth. FIG. 2 shows the structure of FIG. 1 in which an opening 200 has been milled in this fashion. Present methods rely on manual intervention to stop the milling process when the material boundary has been reached.
Another reason for the need to detect material boundaries relates to the use of prior-art gas-assisted etching. Gases are injected near the surface of the semiconductor device during the milling process to increase the efficiency of removing a specific type of material. As the boundaries between different materials are traversed, the type of gas injected is changed to conform to the requirements of the new material; that is, a different gas is used for each material or class of materials.
Systems for the treatment of integrated circuits and the like with a focused-ion-beam (FIB) are known. See, for example, U.S. Pat. No. 5,140,164, the content of which is incorporated herein by this reference. A FIB system commercially available as the "IDS P2X FIBstation" from Schlumberger Technologies, Inc., San Jose, Calif., has a gas manifold and capability for changing from one type of injection gas to another
Material boundaries between different semiconductor layers may be detected during the ion milling process using a variety of known methods. One such method is to characterize the milling process and then to estimate the time to reach the desired end point. The process is characterized by manually controlling the milling of representative samples of a device, noting parameters such as beam current, gases used, and milling time needed to pass through each layer. The process is then repeated on a similar structure using the same parameters, relying on milling time to reach a desired layer of the structure. If the concentration of ions in a given area and the etch rate properties of the material being milled are known, the time needed to mill through a layer of known material and thickness to reach a layer below it can be mathematically predicted. FIG. 3 shows, for example, a source 300 producing a FIB 305 to mill through a layer 310 in a region 315 to expose a layer 320. The process of milling material of layer 310 can be characterized, so that the time t to mill through a layer of such material of a given thickness can be predicted.
Another method of detecting material boundary change during milling is to visually monitor changes in the secondary-electron count and manually terminate the operation when a change is observed. See U.S. Pat. No. 5,140,164 entitled "IC modification with focused ion beam system." As the primary ion beam strikes the surface of a device, low-energy secondary ions and electrons are emitted. Each material has a different yield of secondary-electron emission: therefore, transitions between layers are indicated by a change in secondary-electron yield. The secondary electrons are detected and used to produce a FIB image of the area being milled. Changes in the number of secondary electrons are manifested in the image as changes in the brightness or contrast. By monitoring contrast changes in the FIB image, material transitions may sometimes be detected. For example, FIG. 4 shows a source 400 emitting a FIB 405 to mill layer 410 in a region 415 to expose a layer 420, while secondary electrons 425 are detected by a detector 430. Detector 430 produces a signal which is used to generate the FIB image.
Another method of detecting material boundaries is to visually monitor for changes in the secondary-ion count and manually terminate the milling operation when a change is observed. This method of end-point detection uses an electron-beam shower to neutralize charging of the device, and a detector which is electrically biased so as to detect positively-charged secondary ions. Material transitions are detected by plotting the detected secondary-ion intensity versus the ion dosage per unit area (nanocoulombs per square micron). The resulting traces can be interpreted so as to determine where material transitions occur.
Yet another method of detecting material boundaries is to monitor changes in atomic composition, using known detection techniques such as SIMS, Auger or EDX These allow the composition of the material being milled to be determined by analyzing the waste material removed during the milling process. Material transitions are detected by determining when the composition of the material being milled changes.
A further method of detecting material boundaries is to monitor current passing through the stage on which the device is held during milling. A semiconductor device is electrically grounded to an X-Y stage of the FIB system during milling. As the primary ion beam strikes the surface of the device, electrical charge builds up on non-conductive surfaces. When a conductive material is reached, a path to ground becomes available for this built-up charge. This produces a current from the stage to ground. By monitoring and measuring this current while milling a non-conductive layer, it can sometimes be determined when a conductive material has been reached. FIG. 5 shows a source 500 supplying a FIB 505 which mills through a non-conductive layer 510 in a region 515 to expose a conductive layer 520. When conductive layer 520 is exposed, charge which built up on layer 510 during milling is discharged to ground as a current 525 which indicates the conductive layer has been reached.
It is also known to detect material boundaries by providing within the semiconductor structure a marker layer which has optical properties different from the etched or protected layers so as to be optically detectable when exposed by milling. See U.S. Pat. No. 5,395,769 "Method for controlling silicon etch depth." The method depends upon designing the additional layer into the semiconductor structure and is not relevant to FIB milling of devices which do not have such a marker layer.